I think there's a couple of things to consider - it's always better to have good air-gap under pv panels to stop them getting too hot in use, which can cut their efficiency considerably, but if you attach them with a 3 or 4 inch gap, then hurtle down a motorway, you could have problems with the panels trying to lift, and possibly cracking
I reckon there's 2 ways to go - make up some L-section aluminium into mounts as described in this thread
http://www.navitron.org.uk/forum/index.php/topic,1363.15.html, which will give you a good air gap, and enable you to simply remove the panels when in transit, which will entail mounting into the structure (no big deal, tap around for the framework, then screw into it, using silicone as a sealant where needed.
OR you could use "Martin's down and dirty way" - I kid thee not - velcro! - if you have the sort of ally mount that gives a few mm of frame parallel to the panel at the rear, get a roll of male/female industrial type velcro - attach to rear of frame, remove backing off sticky strip the other side, and whack onto trailer roof..........remove for transit
